In the related art, a liquid flow sensor described in PTL 1 is known as a flow sensor for detecting variations in flow rate of a fluid. The flow sensor includes a semiconductor module incorporating two temperature sensors provided respectively on an upstream side and a downstream side of a fluid flow and a heat source. The flow sensor is provided with a pipe that guides a liquid, and the semiconductor module is provided on an outer surface of the pipe via an adhesive such as a thermally conductive paste. The two temperature sensors and the heat source are in thermal contact with the outer surface of the pipe. Such a structure of the flow sensor is employed when contact of the fluid to materials other than the pipe is not desired, or when the semiconductor module is larger than an inner diameter of the pipe and thus cannot be disposed in the interior of the pipe.